1. Field of the Invention
This invention relates to a copper foil for use in the manufacture of printed circuit boards and flexible circuits. More particularly, a composite material includes a copper foil layer and a support layer with a release layer disposed between the copper foil and the support layer. Contacting the release layer with a reactive element at an elevated temperature reduces blister formation during subsequent assembly steps.
2. Description of Related Art
Composite materials used for the manufacture of printed circuits and flexible circuits are described in the above-referenced U.S. Pat. Nos. 6,346,335 and 6,569,543. A support layer, typically formed from copper, is coated with a release layer. The copper foil layer is formed on the release layer, typically by electrolytic deposition. Adhesion between the release layer and the copper foil layer is high enough so that the copper foil layer does not separate from the support layer prematurely, but is sufficiently low that separation of the support layer following lamination does not tear or otherwise damage the copper foil layer. Typical release layers include an admixture of a metal selected from the group consisting of chromium, nickel, titanium, copper, manganese, iron, cobalt, tungsten, molybdenum, tantalum and mixtures thereof and a non-metal selected from the group consisting of oxides, phosphate and chromates of those metals. One preferred release layer is an admixture of chromium and chromium oxide.
For the manufacture of printed circuits, the composite material is laminated to a glass-filled epoxy-type board with the copper foil layer contacting the epoxy board. A combination of heat (typically between 180° C. and 250° C.) and pressure (typically between 200 psi and 300 psi) bonds the copper foil layer to the epoxy board. The support layer and release layer are then peeled away from the epoxy board/copper foil assembly.
For the manufacture of flexible circuits or high temperature application devices, the copper foil is sometimes laminated in the absence of pressure. The copper foil is coated with a polymer dielectric, such as polyimide. The coated composite material is then dried and cured at a temperature of between 180° C. and 400° C., for example 360° C. for 30 minutes in a nitrogen atmosphere at one atmosphere of pressure. After curing, the support layer and release layer are then peeled away from the polymer dielectric copper foil assembly.
It is important that when the support layer is peeled from the copper foil layer that the copper foil layer is defect free. One such defect that has been reported is blisters on the surface of the copper foil layer that previously contacted the support layer. These blisters are believed to originate when gasses are evolved at the support layer/copper foil interface and then expand with increasing temperature during the dielectric curing process. The blisters are typically a few thousandths of an inch in diameter with a frequency ranging from a few to several thousand per square foot. Upon separation from the support layer, the blisters leave depressions in the surface of the laminated copper foil. These small depressions render those regions of the laminate unacceptable for the processing of fine-line circuits.
There remains a need for a process to manufacture flexible circuits that reduces or eliminates the formation of such blisters.